1. Field of the Invention
The present invention relates to an improved method of bonding a semiconductor chip to a dielectric body, such as a header, and more specifically, to providing an eutectic bond between a silicon chip and a ceramic package.
2. Prior Art
Semiconductor fabrication usually requires a bonding of a semiconductor device, such as a silicon chip, to a packaging element. Various bonding methods, well-known in the prior art, are available which teach the bonding of the chip to the package. One general technique employs the use of gold (Au) and a gold/silicon (Au/Si) alloy as a bonding medium for bonding the silicon chip to the package. Such bonding methods are described in U.S. Pat. Nos. 3,316,628; 3,585,711; 3,593,412; 4,454,528; and 3,298,093.
Although pure gold melts at approximately 1100 degreess C., and pure silicon melts at above 1400 degrees C., it is known that a gold/silicon eutectic alloy melts at a temperature of less than approximately 400 degrees C. Therefore, the placing of the one element, gold, in proximity of the second element, silicon, causes a transmigration of atomic particles and when heated to approimately 400 degrees C., a Au/Si eutectic bond forms between the two elements. The above-named patents recite processes which utilize this property of the Au/Si alloy.
In addition, it is well-known in the prior art that an intermediate metal interposed between the silicon and the gold normally operates as a diffusion barrier which limits the amount of oxidation of the silicon. However, when heated, the increase in thermal energy causes a breakdown of the diffusion barrier and allows the Au/Si bonding to occur.
It is also known in the semiconductor industry to prepare a package surface where the bonding is accomplished by deposition of gold on that surface, usually the bottom surface of a cavity in the package. Then, the die which includes a silicon chip, and an intermediate metal layer, is placed into the package cavity wherein the intermediate metal makes contact with the gold deposition layer. Next the die is scrubbed at a temperature near the Au/Si eutectic melting point which causes the Au/Si eutectic bond to occur between the gold deposition layer and the silicon in the die. Alternatively, a gold or gold/silicon preform may be placed between the silicon chip and the gold deposition layer in the package cavity, followed by a die scrub at a temperature near the Au/Si eutectic melting point to achieve the same results.
A problem with using the Au/Si preform is that the system will only accept a certain amount of silicon. When a Au/Si preform is used, the preform contributes a significant amount of silicon to the system and the amount of silicon coming from the die is limited. Therefore, a good bond may not form between the die and the cavity. When a smaller "seed" is used, the benefits of the Au/Si eutectic preform are obtained without the quantity of Si contributed by a large Au/Si preform.
However, as the physical size of the silicon wafers increased, problems were encountered in the basic die attachment process. Die sizes of 0.200.times.0.200 inches and larger have historically been difficult to die attach due to wafer backside variations. Wafer backside variations occur because it is difficult to maintain a uniform thickness of the intermediate metal, and this non-uniformity is multiplied in wafers having dimensions larger than 0.200.times.0.200 inch.
The non-uniformity of the backside causes a non-uniformity of heat transfer resulting in non-uniform breakdown of the diffusion barrier. Therefore, if the temperature is too low, certain surfaces of the die will bond but portions may result in fast solidification (depending on the amount of Si flowing from the Si die) resulting in formation of voids. To compensate, the temperature may be raised to a level which assures diffusion barrier breakdown over the total area. However, temperatures above 500 degrees C. tend to crack the die due to thermal stress during the subsequent cooling cycles of the process. Such high temperatures also produce metal "spiking."
In U.S. Pat. No. 4,771,018 an improved Au/Si eutectic bonding process is described which may be performed at temperatures low enough to prevent voids and cracks, yet provides for a bond over the complete surface area of a semiconductor wafer die having dimensions larger than 0.200.times.0.200 inches.
Specifically, the method described in U.S. Pat. No. 4,771,018 involves placing a pure gold preform on the gold deposition layer within the cavity. Then a Au/Si seed is placed on the gold preform. Next the die with intermediate layer is placed on the seed and the preform, then gently scrubbed at approximately 400 degrees C. The Au/Si seed melts at the eutectic temperature of Au/Si alloy which is approximately 363 degress C. The melted Au/Si forms a liquid layer which acts as a catalyst in transmitting thermal energy from the gold preform to the silicon, wherein silicon atoms combine with the gold preform and form a eutectic bond.
However, the Au/Si seed which is approximately 98%, by weight gold and approximately 2% by weight silicon is a small square shaped piece, approximately 0.040 inches on each side. The small size of the seed, which must be placed inside the packaging cavity and centered on the gold preform, makes it difficult to automate the process. Presently, a person is needed to first pick up a single gold preform for placement in the cavity, then pick-up a single seed from a receptacle and drop the seed into the cavity on top of the gold preform before the die is placed on the seed and preform.
The present invention is directed to a method which uses a ribbon made of gold instead of a gold preform, and a strip of gold/silicon eutectic alloy cladding on the ribbon instead of a gold/silicon alloy seed. The ribbon is stored on a spool from which the ribbon may be unrolled, and cut into pieces approximately 0.350 inches on a side as needed. A cut piece of the ribbon is then placed into the package cavity, on top of the gold deposition layer and the die is placed on the cut piece, then scrubbed at approximately 400.degree. C. The Au/Si cladding melts at the Au/Si eutectic temperature forming a liquid layer and acts as a catalyst as in the technique taught by U.S. Pat. No. 4,771,018 wherein the silicon atoms from the die combine with the gold atoms from the ribbon and gold deposition layer and form a eutectic bond. However, by using a ribbon stored on a spool instead of a separate preform and seed as in the prior art, the method is much simpler to automate. Additionally, since, in effect, a smaller "seed" is used, a smaller quantity of silicon is contributed than that which is contributed by the Au/Si preform of the prior art, increasing the likehood that a good bond will be formed.